1. Field of the Invention
The present invention relates to a vibration damper installed on a subject body for reducing vibration of the subject body. More particularly, the present invention is concerned with such a vibration damper incorporating suitable oscillating means.
2. Discussion of the Related Art
A dynamic damper is widely known as vibration damping means for damping or reducing vibration of a desired subject body such as a body of an automotive vehicle, which is subject to a vibrational load. In recent years, an active-type vibration damper has been proposed as disclosed in JP-A-61-220925 and JP-A-3-292219. Such an active-type vibration damper as disclosed in these publications is adapted to reduce or control the vibration of the subject body, by using an actuator which generates an oscillating force to be applied to the subject body.
However, the conventional dynamic damper is not capable of exhibiting a desired damping effect in an operating environment in which the frequency of the vibration to be damped changes depending upon various factors. Where the subject body is the body of an automotive vehicle, for instance, the vehicle body is subject to various kinds of vibrations which are generated under different running conditions of the vehicle, for example, depending upon changes in the operating speed of the vehicle engine and the running speed of the vehicle.
Further, the vibration damping effect provided by the known dynamic damper is not satisfactory, since it functions as a secondary vibration system adapted to reduce the vibration of the subject body constituting a primary vibration system, by absorbing the vibration energy of the subject body.
In the active-type vibration damping device, it is required to employ an actuator capable of generating an oscillating force which is large enough to assure a high damping effect with respect to the subject body. Where the subject body has a large size and high rigidity, as in the case of the vehicle body, the energy of the vibration of the subject body is accordingly large. Accordingly, the actuator is required to be large-sized, and the electric power consumed by the actuator is inevitably increased. In addition, it is difficult to apply a sufficient oscillating force to the large and highly rigid subject body. Thus, the conventional active-type vibration damping device is not satisfactory in its damping effect.
Another type of known active-type vibration damping device is disclosed in JP-A-6-235438 and JP-A-7-190139, wherein a vibration system is constituted by a mounting member fixed to the subject body, and an inertia mass member elastically connected to the mounting member through a suitable spring member. The vibration damping device includes an electromagnetic drive mechanism for applying an oscillating force to the inertia mass member of the vibration system, so that a large oscillating force is applied to the subject body, by utilization of a resonance action of the vibration system.
In the known active-type vibration damping device utilizing the resonance action of the vibration system described above, however, the frequency of the vibration of the subject body that can be effectively damped by application of the oscillating force to the subject body is limited to a resonance frequency range whose upper and lower limits are relatively close to the resonance frequency of the vibration system. Where the vibration of the subject body has the frequency outside the resonance frequency range, the vibration cannot be effectively damped by application of the oscillating force to the subject body. Therefore, the damping device is not capable of exhibiting a desired damping effect, particularly where the damping device is used for damping the vibration of the body of a motor vehicle, which is generally subject to the vibration whose frequency changes over a relatively wide range.